efi/libstub: Add efi_warn and *_once logging helpers

Add an efi_warn logging helper for warnings, and implement an analog of
printk_once for once-only logging.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200914213535.933454-1-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>